Draw 3D with Python more in front
Thx to @batFINGER, here is the code that works (@ is important for multiplying Quaternion and Vector in Blender 2.8):

rot = bpy.context.space_data.region_3d.view_rotation

# Instead of adding it to the vector that is returned by region_2d_to_location_3d
# I pass it to the method as last argument
dir = rot @ mathutils.Vector((0,0,-1))
dir = dir.normalized() * -2.0

vec = region_2d_to_location_3d(region, rv3d, (x, y), dir)